The present invention relates to a filter circuit comprising a phase advance circuit acting as a high pass filter and a phase retard circuit acting as a low pass filter.
Many electric circuits which act as oscillators or filters are known. One such oscillator is the Wien bridge which uses phase advance/retard principles. The Wien bridge oscillator comprises a feedback amplifier with 180xc2x0 phase shift at a desired output frequency, the loop gain of the amplifier being adjusted so that a self sustaining oscillation just takes place. The feedback is arranged so that the amplitude of the oscillation remains within the linear region of the amplifier.
The Wien bridge oscillator suffers from the disadvantage that it has an attenuation factor of 3. Where the Wien bridge is used as a filter, this increased attenuation means that higher Q filters suffer from a lack of stability.
A further disadvantage of known circuits using phase advance/retard principles is that they commonly require phase shifts of 90xc2x0 which can be difficult to guarantee, particularly over a wide range of frequencies.
Filters generally incorporate resistor/capacitor circuits comprising at least one resistor and at least one capacitor connected such that a time constant of the filter is a function of the electrical characteristics of the resistor/capacitor circuit. Generally resistor/capacitor circuits are implemented using discreet components as it has proved difficult to provide on-chip resistor/capacitor circuits with the characteristics and stability required for filter circuits. Even with discreet components it is still necessary in many applications to provide stabilising circuits to compensate for example for variations in resistance and capacitance resulting from fluctuations in temperature. As a result conventional filters tend to be relatively complex, bulky and expensive.
It is known that resistance and capacitance components can be provided on-chip, but although it is possible to produce such components in batches in which the component values are consistent as between all members of the same batch, it is difficult to produce components with electrical characteristic values which are close to a predetermined desired value. Furthermore, it is not possible easily to produce on-chip components with high values of resistance and capacitance in a reliable manner, and it is not possible to stabilise the electrical values of such components without extensive additional circuitry. As a result it has not proved possible to provide filters with on-chip resistor/capacitor circuits which determine timing constants of the filter in an economic manner.
It is an object of the present invention to obviate or mitigate some or all of the problems outlined above.
According to the present invention, there is provided a circuit comprising a phase advance circuit acting as a high pass filter, a phase retard circuit acting as a low pass filter, and an amplifier connected in series between an input and an output, a positive feedback loop being provided between the input and the output, wherein the electrical characteristics of the phase advance and phase retard circuits are such that interaction between the phase advance and phase retard circuits is substantially prevented.
Interaction between the phase advance and phase retard circuits can be achieved for example by incorporating simple output buffers in those circuits. The phase advance and phase retard circuits themselves may be extremely simple, for example the phase advance circuit may comprise a series capacitor and a shunt resistor whereas the phase retard circuit may comprise a series resistor and a shunt capacitor.
Preferably, interaction between the phase advance and phase retard circuits is avoided by providing each of those circuits with internal feedback paths which provide low output impedances. In such an arrangement the phase advance and phase retard circuits may incorporate integrator circuits. The internal feedback paths may be achieved by incorporating closed loop negative feedback configurations.
The amplifier may be in the form of a multiplier responsive to a Q setting input. The phase advance and phase retard circuits may comprise resistive and capacitive components and multipliers configured to compensate for variations in the values of those resistive and capacitive components. A calibration circuit may be provided which also incorporates resistive and capacitive components and provides a calibration output to the multipliers of the phase advance and phase retard circuits, the calibration circuit being configured such that the calibration output is responsive to variations in the values of the resistive and capacitive components of the calibration circuit.
Preferably the resistive and capacitive components of the phase advance circuit, the phase retard circuit, and the calibration circuit are substantially identical. The resistive and capacitive components of the phase advance circuit, the phase retard circuit and the calibration circuit may be fabricated on respective chips. The calibration circuit may comprise an oscillator, a differentiator, a rectifier and a low pass filter, the differentiator incorporating the resistive and capacitive components of the calibration circuit. The multipliers of the phase advance and phase retard circuits may also be connected to receive a filter frequency setting input in addition to the calibration output of the calibration circuit.